Conventionally, metal tubes such as steel tubes or aluminum pipes, or corrugated tubes made of resin are used as cable protecting tubes. With such cable protecting tubes, problems may occur such as the influence of noise generated by the cable housed in the protecting tube or the influence of external noise on the cable inside the protecting tube. For example, in hybrid automobiles, in order to protect the cable that supplies three-phase AC output from the inverter to the drive motor, a protecting tube is provided in the lower portion and the like of the vehicle body so as to fit the shape of the vehicle body. However, in this case, noises generated by the cable disrupt the radio and the like and, therefore, shielding is necessary.
Protecting tubes made have been proposed that include a metal tube made of metal, for which the outermost layer is made of stainless steel to improve durability and the other layers are made of iron (see Japanese Unexamined Patent Application Publication No. 2007-81158A).
Additionally, corrugated tubes have been proposed in which a metal layer is plated on a corrugated tube made of resin (see Japanese Unexamined Patent Application Publication No. H09-298382A).
However, the metal tube in Japanese Unexamined Patent Application Publication No. 2007-81158A is made of metal and, thus, has a problem of weight. If the thickness of the metal tube is decreased to reduce the weight, the bent portion may collapse and flatten when bending the tube, thereby making it difficult to maintain the predetermined internal diameter. Additionally, since a certain thickness is required, a large processing machine is needed for the bending. As such, workability during the product shaping process is not always good.
Also, particularly in electric automobiles, electromagnetic shielding tubes are usually disposed in the lower portion of the vehicle and, as such, corrosion resistance against moisture and external damage resistance against flying rocks and the like are required. However, with the metal tubes described above, there is a high possibility of dents being formed when flying rocks or the like collide with the outer surface of the tube. Additionally, costs increase when using stainless steel, such as in Japanese Unexamined Patent Application Publication No. 2007-81158A. Moreover, with such protecting tubes, not only is there a possibility of water becoming adhered to the outside of the tube, but there is also a possibility of water becoming adhered to the inside the tube due to condensation and the like. Consequently, forming a stainless layer only on the outermost layer does not completely solve the problem of corrosion.
Additionally, even when using a method in which a metal layer is adhered to the corrugated tube by plating as described in Japanese Unexamined Patent Application Publication No. H09-298382A, there are problems with peeling, corrosion, and the like of the plating.
With electric automobiles in particular, due to the fact that substantial levels of noise are generated, there is a possibility of other electrical components being affected. However, with the method described in Japanese Unexamined Patent Application Publication No. H09-298382A in which a metal layer is formed on a corrugated tube made of resin using an electroless plating method, there is a limit to the shielding properties that can be obtained from the thickness of the metal layer. Consequently, it is difficult to achieve both high shielding properties and adhesion between the metal layer and the resin layer.
In contrast, composite tubes have been proposed in which resin layers are formed as the inner and outer layers of a metal layer. With a configuration in which the inner and outer layers are resin layers and the intermediate layer is a metal layer, the metal layer is not exposed on the inner surface and the outer surface of the electromagnetic shield tube and, thus, corrosion of the electromagnetic shield can be prevented. Additionally, flattening of the metal layer when bending the electromagnetic shielding tube is prevented due to the resin layers.
Additionally, the outer layer is removed and the metal layer is exposed at an end portion of the electromagnetic shielding tube in which the inner and outer layers are resin layers and the intermediate layer is a metal layer. A flexible conductor such as braided wire or the like is arranged on and connected to the exposed metal layer. The flexible conductor is arranged on the outer periphery of the metal layer and, in this state, is tightened from the outer periphery by a ring member. As a result, conduction can be achieved between the metal layer and the flexible conductor, and the flexible conductor can be connected to the metal layer.
When using an electromagnetic shielding tube in, for example, an automobile or the like, the electromagnetic shielding tube is installed in an environment that is subject to large changes in temperature. Accordingly, the inner layer, the metal layer, and the outer layer may contract or expand due to changes in temperature.
If the inner layer and the metal layer contract, the metal layer will deform in a direction in which the diameter thereof decreases. Particularly, deformation in the diameter reduction direction of the inner layer may progress even after the thermal cycle due to creeping of the inner layer. As such, even in cases where the diameter reduction amount of the metal layer alone due to thermal contraction is small, the metal layer will be pulled toward the center and deformation in the diameter reduction direction will increase due to the contraction of the inner layer, which has a coefficient of thermal expansion (coefficient of linear expansion) greater than that of the metal layer. Additionally, even if the inner layer and/or the metal layer has not deformed, the push back force of the metal layer against the ring member decreases due to changes in stress in the inner layer. Thus, due to the contraction of the inner layer, contact between the metal layer and the flexible conductor connected to the outer periphery of the metal layer may worsen and contact resistance at the connection portion may increase.
Furthermore, the flexible conductor connected to the outer periphery of the metal layer may fall off due to reductions in the diameter of the metal layer. Additionally, when the inner layer of the composite tube contracts due to changes in the temperature, the repulsive force from the metal layer generated when tightening the ring member on the metal layer weakens. As a result, the flexible conductor connected to the outer periphery of the metal layer may fall off.
Thus, when the electromagnetic shielding tube is installed in an environment that is subject to significant changes in temperature, the exposed portion of the metal layer may be affected by differences in the physical properties of the inner layer and the metal layer (e.g. the difference in the coefficients of thermal expansion described above, and the like), and the contact resistance between the metal layer and the flexible conductor may increase.